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A capacitor would tend to delete the signal by smoothing the AC into closer to a DC signal. Fundamentally the AC signal from the alternator VR is not designed to be a tach signal and the fact that it sort-of works is basically a coincidence - it's designed to send alternator RPM To the LBCD and...

Ah - yes so the AC terminal basically outputs alternator RPM to the LBCD and it judges battery state (and thus infers load) based on voltage. But as you pointed out it's a Niehoff spec and we don't know how many pulses per rotation - plus it's alternator RPM not engine RPM. Not really easily...

The alternator AC post is a voltage regulator output that indicates alternator load to the LBCD. It is not a tach signal. The ECM does provide a "complimentary" tachometer drive signal other than the J1587/J1939. On the C7 (I believe it should be the same on the 3126 - communication and...